The invention relates to a light transmitter, a light receiver and a measuring device with the use of such a light transmitter or light receiver for measuring optical properties of transparent substrates.
There are light transmitters or light receivers comprising a hollow body having a highly reflecting and diffusely dispersive, i.e. white, inner surface, a light source and/or light sensor arranged inside the hollow body and light exit opening and a light entrance opening at a distance therefrom. Apart from the light transmitter, which emits diffuse light for illuminating the substrate to be measured, the measuring device largely comprises a light receiver, which is arranged in the optical path of the light, which is emitted from the light transmitter and has passed through the substrate or has been reflected by the substrate.
Usually, collimated radiographic light is aligned on the sample for measuring the transmission and reflection properties of various transparent media like glass, films, coatings or glass filters. Subsequently, the light passing through is once again displayed in an optical characteristic and analyzed in accordance with the measuring task.
This measurement geometry fails to work in the case of concave objects like spectacle lenses, coated lenses or in the case of dispersive, randomly deflective substrates i.e. radiation without intensity loss, like diffusion disks for signal systems. Here, as also in the case of cloudy liquids, it is necessary to measure with diffuse light. In this case, the requirements are extremely customized and depend on properties of the objects. The design generally comprises a light source, arranged in an integration sphere or integrating sphere with its exit opening facing a light receiver that absorbs the emerging light. Likewise, it is also possible to collimate the optical path of a light source and align it on the sample opening of the integrating sphere. In both the arrangements, the device to be measured is introduced in the optical path close to the sphere opening.
An integration sphere or integrating sphere is a hollow sphere having an inner surface with absolutely matte reflection properties. The light of a light source arranged inside the sphere is multiply reflected so that each surface section of the interior surface as well as an exit opening is illuminated equally brightly and its luminous density is proportional to the total luminous flux.
Depending on the direction of the optical path, the substrate along with its surface section to be measured positioned directly at the exit opening or incidence opening of the integration sphere is either diffusely illuminated, i.e. penetrating from a number of various directions or the light dispersed in the substrate is completely absorbed with the sphere. According to each of the selected measurement geometries, a light receiver or a light transmitter is arranged across the substrate facing the exit or incidence opening of the sphere with a defined angular orientation.
E.g. description of a measuring device is given in EP 0 458 223 A2, which uses an integrating sphere as a light receiver for measuring absorption of transparent samples with irregularly arranged surfaces. The samples to be measured are arranged in the integrating sphere automatically or in a volume linked with the sphere, whose surface indicates the same multiply dispersive properties like the inner surface of the sphere. However, such an arrangement is suitable only for small sample geometries.
During the manufacture and/or quality control of optical products, it is often necessary to determine their optical properties for e.g. the reflection and transmission characteristics, and monitor in-situ in order to control the manufacturing process. This is especially the case, if, in a coating process, thin coatings with great uniformity, defined coating thickness and defined optical properties are to be applied on laminar substrates.
For instance, as the angle-dependent transmission of substrates is modified by applying thinner, sputtered coatings for instance, the coating growth is to be observed and controlled from the point of view of quality and thickness by means of the in-situ measurement of transmission during the manufacturing process. The dispersion in the thin coatings itself is in this case insignificant.
In order to measure the reflection and transmission of the coated substrate, photometers are used in the coating chamber, for instance, for the viability of short optical paths, which capture the monochromatic transmission and/or reflection signal of the substrate and a reference signal of the light source of the photometer.
The measuring device described in DE 10 2005 010 681 A1 is suitable for plasma or ion beam-aided processes, where the optical path is clearly extended on account of the significantly great distance of the coating, ion or plasma source from the substrate and on account of the protection required by the measuring device against spurious material deposits. Even in this device, the substrate to be measured intersects the optical path between a light source and a light receiving unit and in any case, there exist high standards in the adjustment of the light transmitter in particular. The protection of the measuring device against the coating source takes place in the last-mentioned device by means of a diaphragm.
A measuring device for the measurement of transmission and reflection for the purpose of the quality control of tape-like paper is described in GB 2 147 413 A. An integrating sphere for measuring the reflected light and light emitted through the substrate is arranged above and below the substrate.
However, the use of an integration sphere as a light transmitter or light receiver for measuring dispersive substrates, for instance, is advantageous neither for measuring in a coating chamber nor for the in-situ measurement of a continuous manufacturing process on account of the susceptibility of such a measuring arrangement to damage and its high costs. Furthermore, in the case of thicker substrates with a high degree of transmission and two reflecting surfaces, the face-to-face arrangement of the light receiver and light transmitter leads to the falsification of the measurement results due to the parts entering into the light receiver, which would be repeatedly reflected between the substrate surfaces.